Image processing systems such as copying machines, printers and image scanners are being required for high-speed processing of a growing quantities of information generated by modern society. An image processing system is usually equipped with an automatic document feeder to provide a continuous input of documents from a stack of such documents, and a paper stacker which stacks the processed documents as they exit from the system.
Referring now to FIGS. 1 and 2, there is shown a prior art paper stacking apparatus 20. The stacking apparatus 20 comprises a conveyor belt 22, support rollers 24 and a stacking tray 26. FIGS. 1 and 2 also show a portion of an image processing machine 40 with exit rollers 41 that feed documents 50 onto the paper stacking apparatus 20. The paper stacking apparatus 20 is coupled to the image processing machine 40. The conveyor belt 22 is driven in a direction shown by an arrow A by a conventional motor (not shown). The stacking tray 26 is attached to the paper stacking apparatus 20 at an angle to facilitate removal of the documents 50 after they are stacked. Stacked documents are designated with the numeral 50a.
The documents 50 which emerge from the exit rollers 41 are conveyed along the conveyor belt 22 at a speed that is slower than the speed at which the documents 50 exit the image processing machine 40. Because of this speed differential, the emerging documents 50 are conveyed with a leading edge of each the document 50 overlapping a trailing edge of the document 50 in front. The documents 50 thus conveyed are pushed up onto the stacking tray 26. As a result of the overlapping, the documents 50 are stacked up on the stacking tray 26 in the correct order.
In order to assure a proper overlapping of the documents 50, it is necessary to feed the documents 50 from the image processing machine 40 at a substantially constant rate and with a prescribed interval or space between the documents 50. If this prescribed interval is not maintained, a stream of the documents 50 exits with intermittent overlapping. This condition is shown in FIG. 2. One of the documents 50, designated as 50b, is shown in FIG. 2 being pushed up against the lower edge of the stacked documents 50a already on the stacking tray 26. When this condition occurs it is possible that the document 50b will be forced into the midst of the stacked documents 50a. When this occurs, the documents 50 are not stacked in the order in which they exited the image processing machine 40.
It is desirable therefore to provide a document stacking apparatus which can maintain a correct order of stacked documents emerging from an image processing machine when the documents are not overlapped.